The problem of separating particles from suspensions is encountered in many industries. In the treatment of raw primary sewage it is frequently the practice to employ a coagulant to flocculate the sludge or solids in a settling basin so that the sludge may be filtered, preferably with a rotary drum vacuum filter. The effluent from the settling tank and water from filtration can be further treated in a secondary treatment process. The filter cake is usually incinerated or otherwise disposed of. For the cake to be incinerated, it is necessary for it to have at least 30% solids to sustain combustion. Some of the chemical coagulants presently utilized in the treatment of raw primary sludge prior to vacuum filtration are lime, ferric chloride, and certain acrylic-type polymers.
Starch, because of its availability and low cost, is used as a flocculating agent in the mining and paper industries. It can also be utilized as a sludge flocculant, although it must be modified to be effective to solve some of the problems of sewage treatment. The starch is treated so as to be positively charged which causes the sludge to flocculate. The sludge is negatively charged and can be flocculated using the cationic starch product of this invention. However, an anionic polymer usually is added to the sludge before the cationic starch polymer is added to improve the flocculation efficiency.
Flocculation is believed to result from one or more of three general mechanisms, namely: a neutralization of the particles; secondly, adsorption of long chain polymers on the surface of the solid particles which form large agglomerates; and thirdly, entrapment of the particles by the formation of a large floc.
In co-pending application, Ser. No. 335,518, a process is described for preparing a quaternary ammonium alkene halide ether of oxidized starch to be used as a flocculating agent. Hunt Pat. No. 3,624,070 (owned by the assignee of this invention) teaches the preparation of granular gelatinizable quaternary ammonium starch ethers from starch and an amine alkene halide.
In both application Ser. No. 335,518 and the present invention, the modified starches are gelatinized prior to the reaction with the amine alkene halide so that a more random-type of substitution is obtained which increases the flocculating effect of the starch ether.
Application Ser. No. 335,518 teaches the use of an oxidized gelatinized cationic starch as a flocculating agent. The starch used is preferably dent corn starch, but other non-waxy starches such as wheat, potato, sago, and rice can be used. The present invention teaches the use of an acid treated waxy maize cationic starch as a flocculating agent. The starch used is preferably waxy maize starch, but any waxy variety or any type having the same characteristics can be used. This product is competitive with other commercial cationic polyelectrolyte flocculation agents in performance and is more economical than most. The primary object of this invention is to provide a process for the preparation of cationic amine alkene halide ethers of acid treated, waxy maize starch of unusually high flocculating capacity. A further object is to prepare a new and useful flocculating agent in the treatment of primary sewage sludge in a simple and economical manner. Another object is to produce a flocculating agent that will flocculate most types of sludges and handle the daily varying characteristics of the sludge without becoming totally ineffective. These and other objects and advantages will become apparent hereinafter.
This invention comprises the use of a cationic acid treated gelatinized waxy-type starch as a flocculating agent for primary sewage sludge.